A team of chemical weapons experts is expected to arrive in the Syrian town of Douma on Wednesday to probe an alleged poison gas attack, Russian officials told reporters.

“Tomorrow (Tuesday) the security services of the United Nations ... will test the routes. And on Wednesday is when we plan the arrival of the OPCW experts,” a senior Russian official said at a press conference in the Russian embassy in The Hague, explaining the roads were still being cleared of mines.

On Monday, Washington and London accused Moscow of blocking the nine OPCW inspectors from reaching the alleged gas attack site and said Russians or Syrians may have tampered with evidence there.

The OPCW Fact-Finding Mission, if it goes ahead, will collect samples and look for clues to determine whether internationally prohibited chemical weapons were used, though it will not assign blame.

Here is an overview of their objectives and challenges:

Samples

A priority will be collecting samples, both environmental (such as soil) and biomedical (such as blood) from victims and the scene of the attack. The samples will be returned to the OPCW's main laboratory in the Netherlands. They are generally split, up to four times in some cases, and sent to independent national laboratories affiliated with the OPCW.

The inspectors are bound to a strict chain of custody procedure, which means they usually must be present when a sample is taken and it must remain in their possession. In the past, the OPCW has worked with medical staff from the World Health Organization, which has provided samples from victims and witnesses, and would be asked to do the same in the case of Douma. These could be blood, urine or tissue samples.

Evidence

By the time environmental samples are taken in Douma it will be well over a week since the alleged attack. This will make it more difficult to collect evidence than a normal crime scene, where authorities often arrive within hours. The OPCW team will work as quickly as possible to collect any samples of chlorine and sarin - which it previously found to have been used in the civil war - or other poisonous chemicals.

Chlorine dissipates extremely rapidly and in some cases is no longer traceable after a day. Other chemical compounds, such as sarin, might be found days or even weeks later.

The inspectors will also be looking for other evidence, such as canisters, rocket or bomb fragments, impact sites and craters and will take photographs and video of them. The delivery mechanisms often contain traces of chemicals.

Witnesses

Witnesses have spoken of hearing the whistle of barrel bombs falling from the sky. These too have been widely used in the Syrian war, as previously documented by the OPCW's Fact-Finding Mission. Some carried canisters of chlorine and an explosive charge. These too, if found, could be proof of a chemical attack.

The team will interview emergency responders, survivors, medical staff who treated victims and other witnesses to determine whether they suffered from symptoms associated with chemicals. These might include suffocating, foaming at the mouth, constricted pupils, convulsions and involuntary urination or defecation.

Safety risks

Fighting between government forces and rebels in Douma, a town in the Ghouta region east of Damascus, has ceased but security risks remain. Inspectors have twice come under fire while trying to get to the site of chemical weapons sites in Syria.

In August 2013, they were shot at by a sniper near eastern Ghouta, where hundreds of people had been gassed with the nerve agent sarin.

In May 2014, a convoy of vehicles carrying inspectors was hit by explosives and automatic gunfire while travelling to the northern town of Kafr Zita in Hama governorate. They were briefly held captive. No one was seriously wounded in either attack. No culprits were identified.